1. Field of the Invention
This invention pertains to the field of chemical processing involving at least one processing step which is sensitive to the presence of at least one component contained within the stream to be processed. More particularly, the present invention relates to a process which economically and advantageously integrates the means for removing the deleterious component with the sensitive processing step by the use of a sorbent which is capable of selectively removing the at least one deleterious component at sorption conditions which enable the stream to be in the vapor phase for subsequent introduction to the sensitive processing step which is also carried out in the vapor phase.
2. Discussion of Related Art
There are many chemical processes in which there is at least one processing step which is sensitive to at least one component contained within the feedstream to the process or to a component which is generated within the process upstream of the at least one sensitive step. Generally, the presence of the sensitive step necessitates the removal of all or most of the at least one deleterious component prior to its being introduced into the sensitive processing step.
These sensitive processing steps may include essentially all aspects of unit operations involved in chemical engineering practice. Thus, there are many chemical processes which cannot tolerate the presence of particular constituents which may be contained within the feedstream. For example, one such process involves the use of membranes for separating methane from natural gas where the presence of condensibles, such as, pentane, hexane, or the like, would be detrimental to the membrane. So too, in those chemical reactions where a catalyst is employed, such catalyst is typically sensitive to various chemical constituents as well. Such sensitive catalysts include, for example, an iron oxide catalyst which is used for the formation of ammonia and which is particularly sensitive to carbon oxides. Without the removal of these deleterious components from the reaction zone, the catalyst will be poisoned, the reaction will not proceed, or proceed very poorly, or totally undesirable side reactions will take place.
Chemical reactions are not the only place in which the presence of certain components causes detrimental results. Thus, when using ion exchange resins, for example, it is frequently necessary to remove certain components from the stream to be processed prior to its being introduced into the ion exchanger. The presence of certain components within the feedstream could very well interfere with the ion exchange process or even destroy its utility completely. More specifically, when ion exchanging water to replace calcium ions with potassium ions, for example, the presence of sodium ions within the fluid stream would be detrimental to the ion exchange process requiring that the sodium ion be removed upstream of the process.
Even in certain distillation steps, particularly during azeotropic distillation, the presence of certain components within the fluid stream to be processed may be deleterious to the successful separation of the azeotropic solution. Again, this necessitates the removal of these constituents prior to the distillation step. The same holds true for still other unit operations, such as, irreversible absorbtion when using zinc oxide, for example, and the like.
No matter which sensitive processing step is involved, it is readily apparent that steps must be and are taken to remove the deleterious components from the stream prior to such stream entering the sensitive step.
There are many chemical operations in which the sensitive processing step is carried out while the stream that is being processed is in the vapor phase. Yet, even though the deleterious component removal steps may immediately precede the at least one such sensitive processing step, and although such removal steps frequently include a step in which the stream to be processed is in the vapor phase, the remaining deleterious component removal steps, however, may be such that they typically require that the stream be condensed to the liquid phase in order to carry out these additional steps. This is true despite the fact that it would clearly be most advantageous and desirable to retain the stream in the vapor phase while being subjected to the deleterious component removal steps in view of the subsequent sensitive processing step which, in this instance, is also carried out while the stream in the vapor phase.
Hence, after having its at least one deleterious component removed, the stream that is being processed, now in the liquid phase, must again be brought to the vapor phase in order to carry out the sensitive processing step. Manifestly, the necessity of having to undergo such repeated phase changes disadvantageously results in the additional expenditure of both capital and operating costs that are associated with carrying out such phase changes.
One particular chemical processing area in which such repeated phase changes are required can be found in the petroleum refining industry, particularly in the hydrotreating processes in which deleterious components are removed from hydrocarbon feedstocks prior to entering such downstream processing operations, as isomerization, catalytic reforming, and the like.
Hydrotreating is a process for catalytically reacting the objectionable elements contained within the feedstock with hydrogen and then removing the hydrogenated form of the deleterious components. Typical objectionable elements removed by hydrotreating include sulfur, nitrogen, oxygen, halides and trace metals, with sulfur and its compounds generally being the most prevalent. Removal of at least the sulfur and nitrogen is required so as to prevent poisoning of the catalysts that are used in isomerization, catalytic reforming, and the like, which are generally both sulfur and nitrogen sensitive. When the hydrotreating process is specifically utilized for the removal of sulfur and nitrogen bearing components, it is usually referred to in the art as hydrodesulfurization.
Such a hydrodesulfurization process is typically conducted on a hydrocarbon feedstream intended for subsequent isomerization containing at least four carbon atoms, particularly light straight run gasoline or light naphthas. Such a feed typically contains sulfur bearing compounds on the order of about 200 ppm of sulfur and nitrogen bearing compounds on the order of about 0 to 10 ppm. As used herein, the term "sulfur" is meant to include sulfur and sulfur bearing compounds and the term "nitrogen" is meant to similarly include nitrogen as well as nitrogen bearing compounds. Such levels of sulfur and/or nitrogen generally adversely affect the performance and life of the isomerization catalyst. Consequently, such a feed is conventionally treated by a hydrodesulfurization step to remove the sulfur and any nitrogen contained therein upstream of the isomerization reactor.
The hydrodesulfurization process, as discussed in, for example, U.S. Pat. No. 3,461,062, the contents of which are incorporated herein by reference, generally involves a pump to transfer the hydrocarbon feedstock to a furnace heater in which the typically liquid feedstream is first vaporized. The now vaporous hydrocarbon stream is then passed into a hydrotreating reactor which catalytically converts, in the presence of hydrogen, the sulfur and any nitrogen present in the feedstream to hydrogen sulfide and ammonia, respectively. A vaporous hydrogen sulfide and ammonia containing feedstream is then withdrawn which must be condensed in order to proceed with the next hydrogen sulfide and/or ammonia removal steps.
In the condenser, generally about about 40% of the gaseous hydrogen sulfide and ammonia is condensed along with the feedstream while the remaining hydrogen sulfide and ammonia leave the condenser as overhead. The now liquid hydrocarbon stream, still containing about 60% to 70% hydrogen sulfide and ammonia, is then passed through a hydrogen separator to remove excess hydrogen and any C.sub.3 and lighter components.
The liquid hydrocarbon stream is then passed through a step which substantially removes the hydrogen sulfide and ammonia components from the stream. Such a hydrogen sulfide and ammonia removal step is typically carried out in a steam stripper column in which the condensed hydrogen sulfide and ammonia contained within the feedstream are removed. In lieu of such a steam stripper column, a hydrogen sulfide and ammonia adsorption bed, or an amine scrubber solution, may also be used provided that the feedstream is cooled further to the proper temperature prior to being introduced to these alternative removal means.
The hydrocarbon feedstream is now ready to be isomerized. However, regardless of whether a steam stripper, an adsorber, or an amine solution was utilized to remove the hydrogen sulfide and/or ammonia, the hydrocarbon stream, now having essentially all of its sulfur and nitrogen content removed, must now be reheated in order to convert it to a vapor once again so that it is in the proper phase necessary for being introduced into the isomerization reactor.
While such a hydrodesulfurization technique for sulfur and nitrogen removal is an effective means for dealing with the presence of sulfur and nitrogen, it is extremely costly. In fact, the conventional practice is to run the hydrodesulfurization unit separately and independently from the isomerization unit which clearly adds to the complexity of the process and to its overall costs. So too, the necessity of repeatedly having to heat and cool the feedstream so as to effect a phase change to accommodate different process steps also adversely affects the economics and efficiency of the overall process.
This is but one example in which a need clearly exists to be able to effectively remove at least one deleterious component from a feedstream in an industrial process which contains a step which is sensitive to this at least one component in an economical and efficient manner.